Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.2 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:L/SI:N/SA:N · score source

EPSS

• CVE-2026-55791: 0.46% estimated exploitation activity in the next 30 days; percentile 40.23 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

1. Overview Craft CMS is vulnerable to Server-Side Request Forgery (SSRF) and Arbitrary JavaScript Injection through the /actions/app/resource-js endpoint. By exploiting the default permissive trustedHosts configuration, an attacker can poison the Host or X-Forwarded-Host header to manipulate the application’s $baseUrl. This bypasses the endpoint’s internal URL validation, forcing the backend Guzzle client to fetch a malicious payload from an attacker-controlled server and reflect it to the client with a Content-Type: application/javascript header. 2. Vulnerability Mechanism (Root Cause) The vulnerability manifests when assetManager.cacheSourcePaths is set to false. The attack chain relies on three structural flaws and insecure defaults: - A. Default Proxy Trust (trustedHosts): Craft’s default GeneralConfig::$trustedHosts is set to ['any']. This allows an attacker to bypass front-end web server (Nginx/Apache) strict Host header validations by simply injecting an X-Forwarded-Host…

Published by the source

19 Jun 2026, 21:15 UTC

Last modified by the source

19 Jun 2026, 21:15 UTC

Source last checked

26 Oct 2026, 14:49 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-c55v-343g-5xff

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# Patch Next advisory brief: CVE-2026-55791

Advisory: Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs
IDs: CVE-2026-55791, GHSA-c55v-343g-5xff
Record: https://patchnext.com/advisories/cve-2026-55791 (revision 1, source checked 26 Oct 2026, 14:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: craftcms/cms 5.10, 4.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist craftcms/cms: affected >= 5.0.0-RC1, < 5.10; >= 4.0.0-RC1, < 4.18; fixed in 4.18, 5.10

## Suggested inspection (read-only)
- Packagist craftcms/cms: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on craftcms/cms.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-c55v-343g-5xff (GitHub-reviewed advisory GHSA-c55v-343g-5xff lists affected ranges: Packagist craftcms/cms >=5.0.0-RC1 =4.0.0-RC1 <4.18)

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